The invention relates to a device for power assistance of uphill maneuvers of a motor vehicle.
Such a power-assistance device is known from Application FR 2828450, the contents of which are incorporated by reference in the present Application. It comprises mainly a means for estimating the slope on which the vehicle is positioned, a means for interpreting the actions of the driver, a means for determining a clutch characteristic curve of the vehicle (or in other words a curve of correspondence of the clutch-pedal position with the torque that can be transmitted by the clutch) and a means for automatically deactivating a braking means of the vehicle. By virtue of these means, the motor vehicle can be held in position on a slope in a state in which the driving wheels are disengaged from the engine and can be caused to move (in the direction of climbing the slope) solely by the actions of the driver on the accelerator and clutch pedals (or in other words without any need for the driver to act on the parking brake). In fact, the functional strategy of the power-assistance device is to release the brake system as soon as the engine torque transmitted to the wheels is sufficient to compensate for the force on the vehicle due to the slope and to gravity.
This type of device offers numerous advantages in terms of simplicity and driving comfort. However, it has two disadvantages, to be explained hereinafter.
Firstly there exists a problem of starting in the case of gentle slopes or level pavement. In fact, according to the strategy indicated above, one waits for a sufficient torque to be transmitted to the driving wheels in order to release the brake. It is understood that this strategy is of such nature to create an odd sensation in the user and the occupants of the vehicle: there is a sensation that the elements connecting the vehicle to the ground are straining before the brake is released. This sensation seems odd inasmuch as it is not felt in a traditional vehicle, in which the parking-brake system is released before the beginning of engagement of the clutch. In fact, this straining of the elements connecting to the ground is not necessary inasmuch as, on a gently sloping or level pavement, the vehicle, under the effect of friction and rolling resistance, the vehicle remains immobile or quasi-immobile, even in a declutched state with the brake system released.
Finally, the power-assistance device mentioned above necessitates a transitional phase, at the beginning of operation in traffic, during which this device is not operational or functions imperfectly, and during which this device records data necessary for construction of a characteristic curve of the torque that can be transmitted by the clutch according to the position of the clutch pedal. This characteristic curve is then used to determine the instant at which release of the brake system must be commanded. Normally this curve is strictly monotonic. Nevertheless, it may happen that, at a given instant during the transitional learning phase, the recorded data define a non-monotonic characteristic curve capable of creating a lack of robustness of the power-assistance device and of interfering with its proper functioning if the curve is eventually used. In addition, this transitional learning phase is quite lengthy, particularly as regards learning of data in the high-torque zone. Consequently, the power-assistance device cannot be used promptly and in particular it cannot be used promptly in the situation in which it offers its greatest interest, or in other words that of a steep slope, where a large engine torque is necessary for starting the vehicle.